The present invention relates generally to position determining devices and more particularly to an apparatus for determining the absolute position of a rotating shaft.
It is a common problem to want to know the position of a device whose position is being controlled by an actuator or some other means. For example, in the controls industry, devices such as valves, each having a valve stem or valve shaft which is rotatable by an actuator, are typically used to control the flow of liquids or various gasses associated with the industrial process. In these applications, it is a common need to know the precise absolute position of the controlled shaft or stem. This information allows for an improved understanding of the process and, subsequently, a more accurate control of the process by a process control system.
A number of prior solutions are known for measuring or determining this aforementioned position. One method is the use of linear variable differential transformers which can provide accurate positional information. However, they require a mechanical linkage to translate the positional information to a sensor and, additionally, consume a relatively high amount of power in their operation.
Slidewire, potentiometers, or other rotary transducers again require a mechanical link to the controlled device and also have the disadvantage of a sliding electrical contact which can cause long-term unreliability as well as having a potential for producing arcing and/or sparking, precluding the use of these devices in volatile environments.
Hall effect transducers, as they are currently used, generally require a mechanical linkage and are generally susceptible to ambient magnetic fields, electrostatic discharge, and other Electro-Magnetic Interference (EMI).
A more efficient form of solution is taught by my U. S. Pat. No. 5,828,060, which issued on Oct. 27, 1998 and is assigned to the same assignee as the present application.
Additionally, all of the aforementioned devices and methods require the use of extensive electronic circuitry to convert the predominately analog positional information to the digital signals normally required by modern computer-controlled industrial process control systems.
Therefore, it is the object of the present invention to provide a reliable position-determining apparatus that does not require a mechanical linkage between the apparatus and a rotatable shaft.
It is a further object of the present invention to provide a position-determining apparatus that has long-term reliability, is immune to EMI, and is cost effective.
It is a further object of the invention to provide a position-determining apparatus that exhibits the benefits of a greatly reduced parts count when interfacing the resultant positional signals to a computer-controlled industrial process control system.